Powder coatings are known. They are mostly employed in situations where environmental problems caused by the solvent content of the coating materials are in the forefront.
The avoidance of high solvent contents in the coating materials is in the forefront, in particular, in the coating of car bodies, in the coating of unformed metal sheets (coil coating) and in the coating of metallic packaging containers (can coating).
DE-A-43 06 102, for example, describes powder coatings based on carboxyl-containing polyesters and compounds having more than one epoxide group per molecule as crosslinking agents, which in addition, as binders, contain one or more acrylicized polyurethane resin(s). Such powder coatings are suitable for the coating of metal sheets which following the coating procedure are to be formed, and couple high hardness with a high level of deformability and flexibility, and also provide good corrosion protection and good decorative properties, for example gloss.
Powder coatings based on carboxyl-containing polyesters, low molecular mass curing agents containing epoxide groups, polyacrylate resins containing epoxide groups, and low molecular mass di- and/or polycarboxylic acids and/or their anhydrides are described by DE-A-43 30 404. These powder coatings are suitable, again, in particular for the coating of metal sheets which are to be formed following the coating procedure, the coatings again coupling high hardness with a high level of flexibility and deformability without cracking, in addition to good optical properties.
DE-A-43 41 235 comprises powder coatings which are suitable for the finishing of car bodies and which contain, as binders, at least one polyacrylate resin which contains epoxide groups and siloxane-containing comonomer constituents. The powder coatings described in DE-A-43 41 235 lead to coatings having good surface properties, for example weathering stability, chemical resistance, scratch resistance and insensitivity to dirt.
DE-A-42 22 194 describes a process for producing a two-coat finish and powder coatings suitable for this process, where the powder coatings contain as binder a polyacrylate resin containing epoxide groups and as crosslinking agent a mixture of a polyanhydride and a carboxyl-containing polyester resin. Such powder coatings are applied as transparent powder coating, in the two-coat finishing of, in particular, automotive components, to a pigmented basecoat and are stoved together with the latter.
On the grounds of solvent avoidance in particular, efforts have been made to an increased extent in recent years to employ powder coatings as described above, for example, for coatings of a wide variety of types. The results, however, have often been unsatisfactory since, in order to achieve a uniform appearance when using powder coatings, relatively high film thicknesses are generally necessary. In addition, the use of pulverulent coating materials necessitates a different application technology, which has the consequence that the plants set up for liquid coating materials can no longer be used.
The aim is therefore to develop powder coatings in the form of aqueous dispersions which can be processed using application technologies for liquid coating materials.
For example, the as yet unpublished German Patent Application P 195 23 084.1 describes aqueous powder coating dispersions based on epoxy resins as binders and on phenolic compounds and/or carboxyl-containing polyesters as hardeners for the coating of packaging containers. The aqueous powder coating dispersion is composed in this case of a solid pulverulent component, comprising binder and hardener, and of an aqueous component comprising a nonionic or anionic thickener and, if desired, further additives specific to dispersions. Such aqueous powder coating dispersions can be applied, in connection with the coating of packaging containers, using the liquid-coating technology employed to date.
The as yet unpublished German Patent Application P 195 18 392.4 claims an aqueous powder coating dispersion which is particularly suitable as a coating for car bodies coated with aqueous basecoat. In this case the aqueous powder coating dispersion is again composed of a solid pulverulent component comprising at least one binder which contains epoxide groups and has a content of from 30 to 45% by weight of glycidyl-containing monomers, and a crosslinking agent comprising preferably straight-chain, aliphatic dicarboxylic acids and/or carboxyl-containing polyesters, and of an aqueous component comprising at least one nonionic thickener and, if desired, further additives specific to dispersions. Such aqueous powder coating dispersions can be applied to car bodies with the liquid-coating technology used hitherto and can, in particular, be stoved even at temperatures of 130 degrees C.